The present invention relates to an active-matrix-type image display device including a plurality pixels arranged in a matrix form, a plurality of data signal lines arranged to correspond to the columns of the pixels, and a plurality of scanning signal lines arranged to correspond to the rows of the pixels, for displaying an image by supplying picture signals from the data signal lines in synchronization with scanning signals supplied from the scanning signal lines. More particularly, the present invention relates to an active-matrix-type image display device capable of providing a gray-scale display using a gray-scale voltage.
A conventional active-matrix liquid crystal display device is a known example of the active-matrix-type image display device. As illustrated in FIG. 15, the conventional active-matrix liquid crystal display device includes a plurality of source lines SL, gate lines GL, a source driver 52 connected to the source lines SL, and a gate driver 53 connected to the gate lines GL. A pixel 60 is provided in each region enclosed by adjacent source lines SL and adjacent gate lines GL. The pixels 60 form a pixel array 51 in the form of a matrix.
The source driver 52 samples a picture signal DAT input in synchronization with timing signals such as a clock signal CKS and a start signal SPS, and applies the picture signal DAT to the source lines SL, after amplifying it if necessary. The gate driver 53 sequentially selects a gate line GL in synchronization with timing signals such as a clock signal CKG and a start signal SPG. When the switching elements in the pixels 60 connected to the selected gate line GL are turned ON, the picture signal DAT applied to the source lines SL is supplied to the pixels 60. Each pixel 60 has an electrostatic capacity, and stores the picture signal DAT supplied.
By the way, in the conventional active-matrix liquid crystal display device, in general, the source driver 52 and gate driver 53 are provided as an external IC. By contrast, in order to reduce the packaging cost and improve the packaging reliability, as shown in FIG. 16, for example, a technique for producing a monolithic structure by forming the pixel array 51 and driving circuits, such as the source driver 52 and gate driver 53, on a single insulating substrate 57 was reported recently. A power supply circuit 55, and a control circuit 54 for supplying various control signals are connected to the driving circuits.
The following description will explain an example of the structure of the source driver 52 for displaying an image corresponding to input digital picture signals, in the conventional active-matrix liquid crystal display device. In this example, a multiplexer-type structure is adopted. According to the multiplexer-type structure, more than one kind of gray-scale voltages supplied from external devices are selected, and applied to the source lines without amplifying the voltages by an amplifier or the like. In order to simplify the explanation, it is assumed that the digital picture signals to be input are 3 bits (8 gray scales).
As illustrated in FIG. 17, the conventional source driver 52 includes one scanning circuit 61 and three latch circuits 62a, 62b, 62c, three transfer circuits 65a, 65b, 65c, one decoder circuit 63, and eight analog switches 64a through 64h, with respect to a single stage, i.e., a single source line SL. Supplied to each stage are 3-bit digital picture signals DAT1 through DAT3, a transfer signal TRP, and eight kinds of gray-scale voltages V1 through V8 as well as the clock signal CKS and start signal SPS. For example, the scanning circuit 61, the latch circuits 62a, 62b, 62c, and the decoder circuit 63 are formed by a shift register, half-bit latch circuits, and eight AND circuits, respectively.
Referring now to FIG. 18, the following description will explain the operation of the source driver 52. Here, in order to simplify the explanation, let""s look at only the three source lines, SL1 through SL3. GL1 and GL2 in FIG. 18 are the waveforms of the scanning signals supplied from the gate driver 53 to the gate lines GL1 and GL2, respectively.
In a horizontal period T1, the source driver 52 fetches the digital picture signals DAT1 through DAT3 when the latch circuits 62a, 62b, 62c are opened and closed in synchronization with an output Q of the scanning circuit 61. In a horizontal flyback period subsequent to the horizontal period T1, the transfer signal TRP becomes active, and the digital picture signals DAT1 through DAT3 fetched in the horizontal period T1 are transferred at a time to the decoder circuit 63 from the transfer circuits 65a, 65b, 65c. The digital picture signals DAT1 through DAT3 transferred to the decoder circuit 63 at a time are decoded into 8-bit signals in the decoder circuit 63, and supplied to analog switches 64a through 64h, respectively. Then, one of the gray-scale voltages V1 through V8 is selected, and output to the source lines SL in a horizontal period T2. Thus, the digital picture signals corresponding to a single horizontal scanning period fetched in the scanning period T1 are output at a time in the next horizontal period T2 by the source driver 52.
However, the above-mentioned conventional structure suffers from the following drawbacks. Namely, in this structure, since a single gray-scale voltage needs to be output to all of the source lines SL at a time, the peak of a current flowing in a gray-scale voltage line (the line to the source driver 52 from a gray-scale power supply for generating the gray-scale voltage) in the period shown by ttrf in FIG. 18 is several tens milliampere. In other words, since the gray-scale power supply is required to produce a driving force satisfying such a condition, the overall power consumption of the liquid crystal display device becomes inevitably very high. Moreover, the component parts of the gray-scale power supply are required to have a high withstanding voltage, resulting in an increase in the production cost.
In resent years, portable information terminals are in widespread use. In such a situation, the demand for a liquid crystal display as a display device of the portable information terminals are increasing because of the thinness of the liquid crystal display device. Since most of the portable information terminals are driven by butteries, the display devices for use in such terminals are strongly required to consume low power.
It is an object of the present invention to provide a low-power-consuming active-matrix-type image display device by reducing, particularly, the power consumption of a gray-scale power supply.
In order to achieve the above object, an active-matrix-type image display device for inputting a digital picture signal, according to the present invention, includes:
a plurality of pixels arranged in a matrix form;
a plurality of data signal lines arranged to correspond to the columns of the pixels;
a plurality of scanning signal lines arranged to correspond to the rows of the pixels;
gray-scale voltage generating means for generating gray-scale voltages of different levels;
a scanning signal line driving circuit for outputting a scanning voltage to the scanning signal lines; and
a data signal line driving circuit for selecting a gray-scale voltage according to the picture signal and outputting the gray-scale voltage to the data signal line, and is characterized by that the data signal line driving circuit has one scanning circuit for each data signal line, and selectively outputs the gray-scale voltage to the data signal lines in synchronization with sequential outputs of active signals from the scanning circuits in one horizontal period.
With this structure, the gray-scale voltage generating means generates gray-scale voltages of different levels corresponding to the number of gray scales of the digital picture signal to be input. The data signal line driving circuit selects a voltage according to the picture signal from the gray-scale voltages of different levels in synchronization with making the scanning circuits corresponding to the data signal lines active sequentially, and outputs the selected voltage to the data signal lines sequentially.
With this structure, since the peak of a current flowing in the gray-scale power supply line for supplying the gray-scale voltage to the data signal line driving circuit from the gray-scale voltage generating means is spread, a smaller driving force is required by the gray-scale voltage generating means as compared to a conventional structure where a single gray-scale voltage is output to all of the data signal lines at a time in one horizontal period. As a result, the power consumption of the gray-scale voltage generating means is reduced, thereby providing a low-power-consuming active-matrix-type image display device.
Moreover, unlike the conventional structure, the present invention does not require a structure for storing and transferring picture signals corresponding to one horizontal period, thereby achieving reduction in the circuit scale. Thus, the structure of the present invention can significantly decrease the area of the circuit, particularly, when the driving circuits are formed using, for example, a polycrystalline silicone thin film. Hence, it is possible to decrease the area of the periphery section (frame section) of the display device, and reduce the number of production steps and the production cost.
Furthermore, the above-mentioned active-matrix-type image display device of the present invention can be constructed so that the gray-scale voltage corresponding to the picture signals fetched in each horizontal period continues to be output to the data signal lines from the data signal line driving circuit until the picture signals are fetched in the next horizontal period.
With this structure, it is possible to use a time substantially equal to one horizontal period as the application time of the gray-scale voltage to the data signal lines, thereby preventing insufficient voltage application to the data signal lines. Consequently, high-quality images are obtained. Besides, in general, it is possible to use a sampling transistor for outputting a gray-scale voltage to the data signal lines. According to the above-mentioned structure, since the sampling transistor does not become inactive, it is possible to prevent variations in the electric potential of the data signal line due to outflow of charges accumulated in the channel region.
Additionally, the above-mentioned active-matrix-type image display device of the present invention can employ a data signal line driving circuit having discharge means for supplying a discharge voltage to the data signal lines.
With this structure, the discharge means applies the discharge voltage to the data signal lines in a period of time from a horizontal flyback period to the fetching of the picture signal in the next horizontal period. The application time of the gray-scale voltage to the last data signal line to which the gray-scale voltage is applied last in a horizontal period is shortest. However, since the discharge time within which the discharge voltage is applied to the last data signal line in the horizontal period is long (substantially one horizontal period). Therefore, the discharge voltage compensates for the insufficient application of the gray-scale voltage. It is thus possible to perform sufficient voltage application to all of the source lines, and provide high-quality images.
As the discharge voltage, it is possible to use on e of the gray-scale voltages generated by the gray-scale voltage generating means.
With this structure, since one of the gray-scale voltages generated by an existing gray-scale power supply is used as the discharge voltage, there is no need to additionally provide a power supply for generating the discharge voltage. Accordingly, it is possible to perform sufficient voltage application to all of the data signal lines without increasing the power consumption and circuit scale.
Furthermore, the discharge means can include a latch circuit which inputs the discharge signal and picture signal and is set or reset when the discharge signal is active, and a selecting circuit for selecting and outputting one of the gray-scale voltages according to the output of the latch circuit, and be constructed so that the latch circuit outputs a signal for selecting a gray-scale voltage used as the discharge voltage to the selecting circuit when the discharge signal is active, and outputs a signal for s electing the gray-scale voltage corresponding to the picture signal to the selecting circuit when the discharge signal is inactive.
With this structure, when the discharge signal is active, the latch circuit is set or reset by the discharge signal, then a signal for selecting the gray-scale voltage to be used as the discharge signal is output. Thus, one of the gray-scale voltages is selected as the discharge voltage, and output to the data signal lines. On the other hand, when the discharge signal is inactive, a signal for selecting a gray-scale voltage according to a picture signal fetched by the latch circuit is supplied to the selecting circuit, then the gray-scale voltage is output to the data signal lines. Accordingly, it is possible to achieve a data signal line driving circuit having a discharge function with a simple structure using the latch circuit.
The above-mentioned active-matrix-type image display device of the present invention can be constructed so that a switching element formed of a polycrystalline silicone thin-film transistor is provided for each pixel, and the data signal line driving circuit and the scanning signal line driving circuit include the polycrystalline silicone thin-film transistors.
With this structure, since the polycrystalline silicone thin film is used as the semiconducting layer of the switching element provided in each pixel, it is possible to significantly increase the mobility compared to a TFT using a noncrystalline silicone thin film. Accordingly, even when a driving method in which the polarity of the voltage to be applied to the data signal is inverted every frame period or every horizontal period, it is possible to perform sufficient voltage application to a data signal line to which the voltage is applied last in a horizontal period, thereby achieving high-quality displays.
In the case when the polycrystalline silicone thin-film transistor is used as mentioned above, the pixels, data signal line drive circuit, and scanning signal line drive circuit can be formed on a single substrate.
According to this structure, by forming switching elements, etc. using polycrystalline silicone thin-film transistors, the driving circuits can be formed on the substrate whereon the pixels are disposed. It is thus possible to decrease the production cost and the packaging cost, and improve the reliability.
Besides, in this case, it is preferred to use a glass substrate as the above-mentioned substrate, and set the maximum temperature in the process of producing the pixels, data signal line driving circuit and scanning signal line driving circuit at 600xc2x0 C. or lower temperatures.
According to this structure, it is possible to use an inexpensive low-melting-point glass substrate, thereby providing an active-matrix-type image display device at a low cost.
In the above-mentioned active-matrix-type image display device, when the digital picture signal is n bits, the data signal line driving circuit can be constructed using one scanning circuit, n latch circuits, and one data signal line output circuit for each data signal line. For instance, the data signal line driving circuit can be formed by 2n AND circuits, and 2n analog switches.
In this structure, since a transfer circuit that is essential to the conventional structure is not required, it is possible to reduce the circuit scale of the data signal line driving circuit. Moreover, in the case when the driving circuit is formed by using a polycrystalline silicone film that is subject to stricter design rules compared to an LSI, it is possible to significantly decrease the circuit area. Thus, this structure is very effective for the decrease in the area of the periphery section (frame section) of the display device, and reduction in the production cost.
The above-mentioned active-matrix-type image display device can use a resistance type digital-to-analog converter or capacitance type digital-to-analog converter as the above-mentioned gray-scale voltage generating means.
According to this structure, with the use of the resistance type digital-to-analog converter or capacitance type digital-to-analog converter, it is possible to generate gray-scale voltages of different levels from a voltage produced by one voltage generator (or two voltage generators when the resistance type digital-to-analog convertor is used). Consequently, the number of input terminals of the data signal line driving circuit can be reduced, thereby providing a more compact active-matrix-type image display device.
For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings.